ANR is a functionality on eNB that helps eNB build the neighbour cell information dynamically. An eNB supporting ANR will instruct the UEs to perform neighbour cell measurements and send them the report on a given event(as dictated by the eNB) , using which the eNB will build the NRT. For each cell the eNB has, it maintains a NRT(Neighbour relation table). For each NR, the NRT contains the Target Cell Identifier (TCI), which identifies the target cell. For E-UTRAN, the TCI corresponds to the E-UTRAN Cell Global Identifier (ECGI) and Physical Cell Identifier (PCI) of the target cell(it also has PLMNids and TAC)
The serving cell of eNB (with ANR func), may instruct the UE to perform neighbour measurements on Intra-LTE, inter-RAT cells. The NR can also be statically provisioned by the OSS(OA&M) infrastructure.
The eNB will use this information for performing the UE handovers as it knows the PCI and ECGI. The idea is to ensure that the relationship configuration time does not affect the successful handover.